Hydrogeology of Jimha Area
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Date
1994-06
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Addis Ababa University
Abstract
The studied area is located 335 Km. southwest of Addis
Ababa in the southwestern Ethiopian plateau . Over half of the
map-area is underlain by thick basalt flows inter layered with
tuffs and volcanoclastic sediments . The Jimma Volcanics, which
is mainly composed of massive rhyolite in thick flows
alternating wjth trachytes, tuffs, ignimbrites and subordinate
basalts, underlain small area in the eastern and northeastern
part of the basin.
Based on the hydrometeorologic data collected for the last
38 years an attempt has been made to calculate the hydrological
parameters l i ke precipitation, evapotranspiration, and runoff.
The ann ual mean rainfall in the basin is 1476.89 mm . The
values of the annual potential evaportanspirtion and actual
evapotranspiration are 837.51 mm and 822.29 mm respectively.
The only runoff that leaves the studied area comes through Boye
river and is 90 .77 million cubic meter of water. The amount of
water which is actually available to recharge the groundwater
circulation within the hydrogeological basin is 51.14 million
cubic meter .
The hydrogeological character of the outcropping
lithotypes was discussed with particular reference to
infiltration and their water bearing capacity.
The hydr ogeology of the area varies in different parts of
the basin. There is a great potential of groundwater in the
valleys which is partly filled with alluvial sediments while aless significant ameunt exist within the velcanic recks ferming
the uplifted berders ef the valleys.
Determinatien ef the hydraulic parameters like
transmissivity and permeability have been carried eut. Due to.
the absence ef ebservatien well nearby a discharging wells it
was impessible to. calculate the ceeff icient ef sterage, a
significant earameter which measures the velume ef water the
aquifer releases er take into. sterage. Fer basaltic aquifers
a transmissivity value ef 18.39 m2/day and a permeability value
ef 3.31 x 10-1 cm/ sec. have been calculated. A transmissivity
value ef 1.89 m2 /day . and a permeability value ef 1.62 x 10-5
cm / sec. have been co.mputed fer beth alluvial and rhyelitic
aquifers.
Because ef the lack ef bere wells in the seuthwest, seuth,
and seutheast lewer mest part ef the basin a thereugh study ef
the actual variatien ef greundwater level was net pessible.
Fer the ether part ef the basin, apprepriate greundwater level
centeur lines was censtructed on the basis of the available
static level data.
Preper chemical analysis ef beth the greundwaters and
surfaces water carried eut in the basin shew lew tetal
disselved selids. All of the samples have less than 1000 mg / l
disselved selids. The analysed samples also. cempared with the
acceptable standard for human, agr icul tural, and industr ial
use.
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Keywords
Hydrogeology